In a large sized, microwave safe, mixing bowl mix the butter and marshmallows.
Microwave at 30 second intervals until the butter is melted and the marshmallows are soft. They will puff up really big so make sure your bowl is large!
Stir to combine the butter and marshmallows into a slurry.
Pour in the cereal.
Quickly stir to coat the cereal evenly.
Press into a 9x13 pan sprayed with cooking spray.
Let them set while you melt the chocolate.
Melt the chocolate in a small bowl in the microwave at 15 second intervals until melty and creamy.
Drizzle over the rice krispie treats.
Sprinkle with shamrock sprinkles.
Let set at room temperature until the chocolate has hardened.